Recently, use of electricity as a source of energy has increased for addressing environmental problems related to CO2 reduction, depletion of fossil fuel resources and the like. Therefore, for example, electric vehicles utilizing secondary batteries are being actively developed in the automobile industry. In addition, secondary batteries are featured also in view of efficient use of natural energy such as solar or wind energy.
In general, lithium ion secondary batteries are used as the secondary batteries for driving electric vehicles, at present, in view of the relationship between power and energy density. On the other hand, various companies have focused on the development of next-generation batteries in view of increased energy density, output, safety and the like. The next-generation batteries are in the fields with high future growth in the market.
On the other hand, in secondary batteries other than lithium ion secondary batteries, primary batteries, capacitors (condensers) and the like, separators formed from paper, non-woven fabrics, porous films or the like are used. The performances required for the separators are, in general, short circuit protection between positive and negative electrodes, chemical stability with respect to electrolytic solutions, low inner resistivity and the like. The aforementioned requisite performances are universal ones required in separators regardless of types thereof, although they differ in degree in accordance with devices.
Separators of almost all lithium ion secondary batteries use porous membranes formed from a polymer organic compound such as polypropylene, polyethylene or the like. The aforementioned porous membranes possess some characteristics suitable for lithium ion secondary batteries. For example, the following characteristics can be mentioned.    (1) Chemical stability with respect to electrolytic solutions is exhibited, and no fatal failures occur by separators.    (2) Thickness of a separator can be freely designed, and for this reason, separators responding to various demands can be provided.    (3) The diameter of pores can be designed to be reduced, and for this reason, superior lithium shielding properties are exhibited, and short circuit caused by lithium dendrite hardly occurs.    (4) When thermal runaway of lithium ion secondary batteries occurs, the initial thermal runaway can be controlled by melting polypropylene or polyethylene and thereby narrowing pores.
However, conventional research for lithium ion secondary batteries cannot identify an underlying cause of an occurrence of thermal runaway. Various companies have studied and proposed a means for avoiding risks of thermal runaway of various materials used in secondary batteries by empirical tools, under present circumstances. Developments of materials suitable for vehicles having increased safety are considered by clarifying the principle of thermal runaway and establishing a common evaluation method thereof hereafter. Problems with respect to safety are expected to be overcome.
On the other hand, the second problem in secondary batteries for use in vehicles is cost. A separator is a material accounting for 20% of the battery cost, and further cost reduction is required under the present circumstances.
For example, in the field of rechargeable transportation units such as electric vehicles, and in the field of portable electronic terminals such as mobile phones, an electrical energy-storage device having an increased amount of storage electrical energy per unit volume is required in order to be operational for a long period of time even with a reduced volume. As an example of the aforementioned electrical energy-storage device, mention may be made of an electrical double-layered capacitor in which an electrolyte dissolved in an electrolytic solution is adsorbed by an electrode, and electrical energy is stored on the interface (electrical double layer) formed between the electrolyte and the electrode.
Main roles of separators in the electrical double-layered capacitors are short circuit protection of electrodes (separatability), non-blocking movement of ions in the electrolytic solution (low inner resistivity), and the like. However, the aforementioned porous membranes have high density, and for this reason, the inner resistivity tends to increase. On the other hand, it is known that non-woven fabrics are used as a separator of a capacitor, but there are problems in that when a fiber diameter is reduced or a fiber density is increased in order to maintain separatability, an inner resistivity increases. For this reason, development of a separator with a reduced inner resistivity is desirable.
There are two major processes for preparing polymer porous membranes of polypropylene, polyethylene or the like, namely a wet process and a dry process. The aforementioned preparation processes have respective characteristics. In the wet process, a plasticizer is added to a polymer such as polyethylene to form a film, subsequently, the film is biaxially drawn, the plasticizer is removed by cleaning with a solvent, and thereby, pores are provided. In this process, there are advantages in that pore size or film thickness can be superiorly adjusted, and response to various demands for all individual types of batteries can be carried out. On the other hand, there is a problem in that the preparation process is complicated, and for this reason, cost increases. In contrast, in the dry process, a polymer such as polyolefin is dissolved, the polymer is extruded on a film, the film with the polymer is subjected to annealing, the annealed film is drawn at a low temperature to form pores at the initial stage, and subsequently, drawing at a high temperature is carried out to form a porous product. In this process, there are advantages in that polymers having different melting points can be laminated, and the process is easy, and for this reason, the product can be produced at a reasonable cost. On the other hand, there is a problem in that sensitive adjustment of pores or thickness cannot be carried out.
A separator using non-woven fabrics formed from synthetic fibers, inorganic fibers or the like, other than the polymer porous films, has also been proposed. Conventional non-woven fabrics include dry types of non-woven fabrics and wet types of non-woven fabrics, and both of these have also been utilized as separators. It is believed that dry types of non-woven fabrics by which uniformity of fiber distribution cannot be obtained has a low effect of isolating electrodes, and for this reason, they cannot be used for lithium ion secondary batteries. On the other hand, wet types of non-woven fabrics have characteristics in that fiber distribution is uniform, as compared with dry types of non-woven fabrics. In addition, a higher porosity can be adjusted, as compared with porous films due to the characteristics of the preparation process, and for this reason, a sheet with reduced impedance can be produced. However, it is substantially difficult to use the dry types of non-woven fabrics in batteries using graphite negative electrodes which are widely applied to lithium ion secondary batteries at present. This is due to the characteristics of lithium ion secondary batteries which produce lithium dendrite at the negative electrode side. The aforementioned lithium dendrite has a property in which lithium dendrite is easily produced on the surface of a negative electrode which many lithium ions in a separator pass. For this reason, in non-woven fabrics in which roughness in a range with several dozen μm order is formed in the sheet itself, the parts at which lithium dendrite is easily formed are course. Therefore, shielding properties of controlling short circuit at the time of producing lithium dendrite may be reduced, as compared with a film type one.
In order to overcome the aforementioned problems, specifying a pore size to a specified range is carried out, as described in Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. H11-040130). However, the pore size depends on a fiber diameter. For this reason, the fiber diameter needs to be reduced in order to control the pore size to a small size. In the present technology, it is difficult to produce fibers with a nano-order size at a reasonable cost. For this reason, even if synthetic fibers which are referred to as ultrafine fibers are used, it is substantially impossible to control the pore size to a size suitable for a lithium ion secondary battery. Therefore, lithium shielding properties cannot be improved.
In addition, a method for producing a non-woven fabric using an electrostatic spinning method as described in Patent Document 2 (Japanese Patent No. 4425576) is proposed. However, the aforementioned method may not be a realistic method, considering production efficiency and that it is substantially difficult to prepare a sheet having a thickness of several dozen micrometers in present production facilities.
On the other hand, many separators of cellulose type are proposed. For example, Patent Document 3 (Japanese Patent No. 4201308) describes that since the hydroxyl groups of cellulose are not electrochemically stable, an acetylation treatment is carried out, and thereby, the hydroxyl groups are stabilized to have an aptitude of a lithium ion secondary battery. However, a separator mainly having cellulose has been used in trials of some lithium ion secondary batteries. For this reason, electrochemical stability of cellulose per se in a lithium ion secondary battery may not be a problem.
Patent Document 4 (Japanese Patent No. 4628764) also proposes a separator using cellulose nanofibers. Only cellulose fibers having a thickness of 1,000 nm or less described in Patent Document 4 are reliably obtained in accordance with a method of utilizing bacteria cellulose as described in Patent Document 4 or the like. However, a method of industrially obtaining cellulose fibers by using bacteria cellulose is not established, and a production cost is unclear. Therefore, the aforementioned method may not be an effective means for producing a sheet at a reasonable cost. In addition, Patent Document 4 also describes a means of utilizing natural cellulose. When natural cellulose is treated to uniformly have a thickness of 1,000 nm or less, fibrillation proceeds. Thereby, properties of maintaining water are increased, viscosity is greatly increased as a raw material for papermaking, and poor efficiency of dehydration is exhibited. For this reason, the aforementioned method may not be an effective means. In addition, Patent Document 4 also describes that the production can also be carried out by a casting method, but the process of forming pores is different from that in papermaking. Nevertheless, Patent Document 4 fails to clearly describe a means therefor or provide a sufficient description therefor.
In addition, papermaking is carried out by using a filter fabric or mesh in a step of forming a sheet. In accordance with this method, the filter fabric face is transferred during dehydration, and for this reason, irregularities of several micrometers are formed at the transferred face side. Therefore, when the separator is incorporated in a lithium ion secondary battery, insufficient adhesion between the separator and electrodes is exhibited, and battery performance may be degraded. Therefore, this is not preferable.
Patent Document 5 (Japanese Unexamined Patent Application, First Publication No. 2010-090486) proposes a sheet in which an oil-based compound is emulsified using fine cellulose fibers, and air resistance is controlled within a specified range. In this method, a method in which opening of pores is carried out by emulsifying the oil-based compound, but the emulsion is broken when moisture is evaporated at a drying step, and thereby, large pores having a size of 1 μm or more are non-uniformly formed in the sheet. As a result, lithium shielding properties are degraded, and short circuit caused by lithium dendrite easily occurs. For this reason, the aforementioned method cannot be used in lithium ion secondary batteries.